While you're right, proving that the OGL is irrevocable will require a legal battle against Hasbro. And while WotC never officially said they're killing the safe harbour, they refused to clarify the situation with deauthorisation despite the community pleading that they do for weeks. For all we now, the deauthorisation is still underway, it's just that the replacement will be a (slightly) sweeter deal.
So given that, isn't it understandable for 3PPs to no longer consider the harbour safe and move to an actually irrevocable license that will be stewarded by a nonprofit group? And even though the ORC cannot be put back in the bottle now, Paizo said they are willing to challenge the irrevocability of the OGL if things get there, so they are going to do what you said anyway.